Determination of cardiac output, arterial blood gases, and other hemodynamic or cardiovascular parameters is critically important in the treatment and care of patients, particularly those undergoing surgery or other complicated medical procedures and those under intensive care. Typically, cardiac output measurements have been made using pulmonary artery thermodilution catheters, which can have inaccuracies of 20% or greater. It has been found that the use of such thermodilution catheters increases hospital costs while exposing the patient to potential infectious, arrhythmogenic, mechanical, and therapeutic misadventure. Blood gas measurements have also heretofore been made. Commonly used blood gas measurement techniques require a blood sample to be removed from the patient and transported to a lab analyzer for analysis. The caregiver must then wait for the results to be reported by the lab, a delay of 20 minutes being typical and longer waits not unusual.
More recent advances in the art have provided for “point-of-care” blood testing systems wherein testing of blood samples is performed at a patient's bedside or in the area where the patient is located. Such systems include portable and handheld units and modular units which fit into a bedside monitor. While most point-of-care systems require the removal of blood from the patient for bedside analysis, a few do not. In such systems, intermittent blood gas measurements are made by drawing a sufficiently large blood sample into an arterial line to ensure an undiluted sample at a sensor located in the line. After analysis, the blood is returned to the patient, the line is flushed, and results appear on the bedside monitor.
A non-invasive technology, pulse oximetry, is available for estimating the percentage of hemoglobin in arterial blood that is saturated with oxygen. Although pulse oximeters are capable of estimating arterial blood oxygen content, they are not capable of measuring carbon dioxide, pH, or venous oxygen content. Furthermore, ex vivo pulse oximetry is commonly performed at the fingertip and can be skewed by peripheral vasoconstriction or even nail polish.
Unfortunately, none of the available systems or methods for blood gas analysis provide for accurate, direct and continuous in vivo measurements of arterial and venous oxygen partial pressures, carbon-dioxide partial pressure, pH, and cardiac output, while presenting minimal risk to the patient.
Coatings and their applications to medical devices have heretofore been described. See, for example, U.S. Pat. Nos. 3,443,869, 4,673,584, 5,997,517 and 5,662,960. Coatings have been employed to maintain lubricity while minimizing complications arising from use of exogenous material in vivo. Certain coatings require reapplication to maintain lubricity and certain lubricious coatings require administration of heparinized saline to maximize immunological tolerance. For devices such as catheters and probes, extraction from a physiological environment for reapplication of a lubricant increases operational costs as well as exposing the patient to heightened risk of mechanical and therapeutic misadventure. Furthermore, reapplication of a coating can compromise the gas permeability of the membrane upon which the coating is applied.